The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at reduction of engine power, for example, as a result of the operation of an air conditioner.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with a desired transmission ratio. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a changing speed which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio.
In such a system, in order to prevent overshooting or hunting at transient state, the convergence of the transmission ratio must be improved as well as the response of the system. Therefore, the desired transmission ratio and the control signal are corrected to obtain an optimum value during any driving condition.
Japanese Patent Laid Open 58-180865 (U.S. Pat. No. 4,566,354) discloses a control system wherein the transmission ratio is corrected to a large value during the cold engine, compared with the warmed-up state.
When an air conditioner is operated, the engine power likewise drops. Accordingly, it is preferable to increase the transmission ratio as in the above-mentioned prior art, thereby compensating the engine power. However, a transmission ratio of the continuously variable transmission is dependent on the engine speed and vehicle speed. Further, engine speed increases at high speed in a large transmission ratio range while the vehicle speed increases at low speed. On the other hand, the engine speed increases at low speed in a small transmission ratio range while the vehicle speed increases at high speed. The transmission ratio changing speed (rate) also changes with the engine speed. Therefore, if the transmission ratio is increased by a predetermined value constant as described in the above prior art, the increased value is too large the large transmission ratio range. Accordingly, the engine speed becomes very high transmission ratio is controlled irrespective of the throttle opening degree so that the engine speed increases needlessly at wide open throttle.
Accordingly, it is preferable to vary the correcting quantity in accordance with the engine speed, actual transmission ratio and throttle position. Since the engine speed changes in proportion to the throttle position, the correcting quantity can be controlled dependent on only two factors, namely, actual transmission ratio and throttle position.